re-encode your footage with a lossless codec like Lagarith (get codec an installer) using virtualdub(/mod)
the files will be large, but they should work. that's why you should take small clips in virtualdub(/mod)

the properties window rounds down.
so 23.987 rounds down to 23 (the closest I could get was some 29.97 fps that got rounded down to 29, but I assume that the same thing happens with 23.987 or whatever it is)